A nexus to a network socket or a component which is capable of I/O operations such as read, write, connect, and bind.

【連接到網絡socket或組件的連接，它能夠進行I/O操作，如讀、寫、連接和綁定。】

A channel provides a user: 【一個channel 能提供：】

• the current state of the channel (e.g. is it open? is it connected?), 【channel當前的狀態，例如 是否開啟，是否連接】
• the configuration parameters of the channel (e.g. receive buffer size), 【channel的配置數據，例如 接收的buffer長度】
• the I/O operations that the channel supports (e.g. read, write, connect, and bind), 【channel支持的I/O操作，例如，讀、寫、連接、綁定】
• the ChannelPipeline which handles all I/O events and requests associated with the channel.【獲取關聯的ChannelPipeline，使用上面註冊的handler處理所有的I/O事件】

1、All I/O operations are asynchronous. 【所有的 I/O 操作都是異步的】

All I/O operations in Netty are asynchronous. 【所有的 I/O 操作在netty中都是異步的】

It means any I/O calls will return immediately with no guarantee that the requested I/O operation has been completed at the end of the call. 【這意味著任何I/O調用都會立即返回，不能保證請求的I/O操作在調用結束時完成】

Instead, you will be returned with a ChannelFuture instance which will notify you when the requested I/O operation has succeeded, failed, or canceled.【相反，會返回一個ChannelFuture 對象，它會在I/O操作成功、失敗、取消時通知你】

2、Channels are hierarchical.【Channel是有 等級/層次 的】

A Channel can have a parent depending on how it was created.【誰創建了它，誰就是它的父channel】

For instance, a SocketChannel, that was accepted by ServerSocketChannel, will return the ServerSocketChannel as its parent on parent().【例如，ServerSocketChannel通過accept()方法接受一個SocketChannel後，調用SocketChannel的 parent()會返回 ServerSocketChannel對象】

The semantics of the hierarchical structure depends on the transport implementation where the Channel belongs to. 【層次結構的語義取決於通道所屬的Channel實現】

For example, you could write a new Channel implementation that creates the sub-channels that share one socket connection, as BEEP and SSH do.【例如，你可以編寫一個新的Channel實現，它創建一個子Channel，它與父Channel共享一個socket的內存，例如BEEP和 SSH do】

3、Downcast to access transport-specific operations.【向下轉型獲得子類的特殊操作】

Some transports exposes additional operations that is specific to the transport. 【某些子類傳輸會提供一些特定的操作】

Down-cast the Channel to sub-type to invoke such operations.【向下轉型成子類傳輸以獲得這些操作】

For example, with the old I/O datagram transport, multicast join / leave operations are provided by DatagramChannel.【例如，UDP傳輸有特定的 jion() 和 leave() 操作，可以向下轉型成 DatagramChannel獲得這些操作】

4、Release resources.【釋放資源】

It is important to call close() or close(ChannelPromise) to release all resources once you are done with the Channel. 【一旦你使用完Channel，必須調用 close() 或 close(ChannelPromise)方法釋放一些重要的資源】

This ensures all resources are released in a proper way, i.e. filehandles.【確保這些資源以一個適當的方式釋放，比如文件句柄】

1 public interface Channel extends AttributeMap, ChannelOutboundInvoker, Comparable<Channel>

• AttributeMap ：簡單理解為一個存放屬性的map
• Comparable ：表明Channel是可以比較的
• ChannelOutboundInvoker ：網絡通信

public interface ChannelOutboundInvoker {
    //綁定本地地址
    ChannelFuture bind(SocketAddress localAddress);
    ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise);
    
    //連接遠程地址
    ChannelFuture connect(SocketAddress remoteAddress);
    ChannelFuture connect(SocketAddress remoteAddress, ChannelPromise promise);
    ChannelFuture connect(SocketAddress remoteAddress, SocketAddress localAddress);
    ChannelFuture connect(SocketAddress remoteAddress, SocketAddress localAddress, ChannelPromise promise);
    
    //解除連接
    ChannelFuture disconnect();
    ChannelFuture disconnect(ChannelPromise promise);
    
    //關閉Channel
    ChannelFuture close();
    ChannelFuture close(ChannelPromise promise);
    //與EventLoop解除註冊
    ChannelFuture deregister();
    ChannelFuture deregister(ChannelPromise promise);
    /**
     * Request to Read data from the {@link Channel} into the first inbound buffer, 
     * triggers an {@link ChannelInboundHandler#channelRead(ChannelHandlerContext, Object)} event if data was read, 
     * and triggers a {@link ChannelInboundHandler#channelReadComplete(ChannelHandlerContext) channelReadComplete} event so the
     * handler can decide to continue reading.  If there‘s a pending read operation already, this method does nothing.
     * This will result in having the
     * {@link ChannelOutboundHandler#read(ChannelHandlerContext)}
     * method called of the next {@link ChannelOutboundHandler} contained in the {@link ChannelPipeline} of the
     * {@link Channel}.
     */
    /**
     * 從channel中讀取數據到第一個 InboundBuffer，
     * 1、觸發 ChannelInboundHandler#channelRead(ChannelHandlerContext, Object)事件，在有數據的情況下
     * 2、觸發ChannelInboundHandler#channelReadComplete(ChannelHandlerContext)事件，handler繼續讀取
     * 如果有read操作已經掛起，則不執行任何操作
     * 實際調用：Channel——>ChannelPipeline——>ChannelOutboundHandler——>ChannelOutboundHandler#read(ChannelHandlerContext)——>read()
     * ChannelHandlerContext繼承了ChannelOutboundInvoker，它的子類實現read()方法，最後ctx.read()。
     * */
    ChannelOutboundInvoker read();
    //寫
    ChannelFuture write(Object msg);
    ChannelFuture write(Object msg, ChannelPromise promise);
    
    //將數據沖刷到Channel
    ChannelOutboundInvoker flush();
    //write + flush
    ChannelFuture writeAndFlush(Object msg);
    ChannelFuture writeAndFlush(Object msg, ChannelPromise promise);
    ChannelPromise newPromise();
    ChannelProgressivePromise newProgressivePromise();
    ChannelFuture newSucceededFuture();
    ChannelFuture newFailedFuture(Throwable cause);
    ChannelPromise voidPromise();
}

public interface Channel extends AttributeMap, ChannelOutboundInvoker, Comparable<Channel> {
    //get屬性
    ChannelId id(); //獲得一個唯一的channelId
    EventLoop eventLoop();//獲得關聯的EventLoop
    Channel parent();//父Channel
    ChannelConfig config();//獲得配置參數
    ChannelMetadata metadata();//獲得元數據
    SocketAddress localAddress();//獲得本地地址
    SocketAddress remoteAddress();//獲得遠端地址
    ChannelFuture closeFuture();//獲得Channel關閉時的異步結果
    ChannelPipeline pipeline();//獲得事件管道，用於處理IO事件
    ByteBufAllocator alloc();//獲得字節緩存分配器
    Unsafe unsafe();//獲得Unsafe對象
    
    //狀態查詢
    boolean isOpen();//是否開放
    boolean isRegistered();// 是否註冊到一個EventLoop
    boolean isActive();// 是否激活
    boolean isWritable();// 是否可寫
    
    long bytesBeforeUnwritable();
    long bytesBeforeWritable();
    @Override
    Channel read();
    @Override
    Channel flush();
}

public interface Channel extends AttributeMap, ChannelOutboundInvoker, Comparable<Channel> {
....  
 interface Unsafe {
        RecvByteBufAllocator.Handle recvBufAllocHandle();//當接受數據時返回它，用於分配ByteBuf
        SocketAddress localAddress();
        SocketAddress remoteAddress();
        void register(EventLoop eventLoop, ChannelPromise promise);
        void deregister(ChannelPromise promise);
        void bind(SocketAddress localAddress, ChannelPromise promise);
        void connect(SocketAddress remoteAddress, SocketAddress localAddress, ChannelPromise promise);
        void disconnect(ChannelPromise promise);
        void close(ChannelPromise promise);
        void closeForcibly();//當註冊失敗時強制關閉連接
        void beginRead();
        void write(Object msg, ChannelPromise promise);
        void flush();
        ChannelPromise voidPromise();//返回一個特殊的可重用的ChannelPromise，它僅作為一個容器不用於操作成功或失敗的通知器
        ChannelOutboundBuffer outboundBuffer();//返回消息發送緩沖區
    }
}

4、Channel簡介